Abstract: In order to study the evolution mechanism of inclusions in the production of X70 pipeline steel by "LF＋VD" process，intensive sampling and analysis were carried out，and the VD furnace deep vacuum process and  calcium treatment process were optimized. The results show that in the deep vacuum process of VD furnace，strong argon stirring for 13 min in the early stage and weak argon stirring for  5 min in the later stage can promote the full floating of large particle inclusions. At the same time，the calcium treatment after the VD furnace broken can control the calcium mass fraction at 0.001 0 %-0.001 4 %，which can promote the full denaturation of inclusions to reach the purpose of low melting point area，so that the qualified rate of inclusions less than or equal to 2.0 in the first inspection of steel plate can be increased from 96.3 % to more than 98.7 %，and the cleanliness of molten steel is improved.

Key words: pipeline steel, cleanliness, VD furnace, inclusions